Water and sewage treatment systems are well known. For more than 50 years, rectangular clarifiers (rectangular concrete tanks) have been used in the treatment and purification of sewage, sewer drain, and drinking water. They are used to remove those materials which are either not biodegradable or cannot be chemically treated during the water purification process. Typically, these clarifiers have been referred to as either primary (4-shaft systems) or secondary clarifiers, (b 3-shaft systems). A primary clarifier removes: (1) non-biodegradable solids (i.e. grit) that precipitate to the bottom of the tank where they are collected, and (2) scum which is collected from the top surface of the water. A secondary clarifier removes the non-biodegradable solids that precipitate to the bottom of the tank that have not been removed during primary clarification. This can be accomplished in the following manner:
Systems include motor-driven sprockets which move two parallel chains which have flight members attached thereto. The flight members scrape the floor of rectangular clarifier tank in one direction, gathering all solid waste materials which have fallen to the floor of the tank and moving them to a cross collector located at one end of rectangular tank. The solids collected in the cross collector are then pumped to a location for additional treatment or final disposal.
Both primary and secondary clarifiers perform this function. Primary systems perform an additional function. Waste materials which are not biodegradable and have specific gravity less than water (i.e., soap or oils) float to the top and sit on surface of water. This solution is commonly referred to as scum. 4-shaft systems have flights protruding half out of the water, running in the opposite direction (from those flights moving solids on the bottom of tank), supported on return rails. The flights move in one direction on top surface of water and push this waste into an element called a scum collector, where these waste materials are removed to a location for additional treatment and final disposal. A number of different stages are generally used in the overall treatment system. Historically, all mechanical components in rectangular clarification systems were metallic, driving first-grade redwood flights. These components typically consisted of steel drive and idler shafts connected to babbitted steel wall bearings which are set in steel wall brackets. Steel chain to which wooden flights are attached are driven by steel sprockets. All return rails attaching rail brackets and floor rails are made from steel. Wear shoes have also been made from steel. The chemicals and oxygen used to treat the water and the general water environment cause severe corrosion of all steel components. It is common for the steel components to be porous, allowing grit and scale to pack in the surface of metal parts. The combined effects of these problems cause severely accelerated wear of all metallic components. Steel drive and steel idler shafts become permanently stuck in steel wall bearings and will not rotate, which causes system failures. Metallic parts are energy consuming. They weigh six times more than plastic. Plastic components often have more than double the life of steel parts. Because of their light weight, they can be installed or repaired at one-third the cost of steel components.
Many treatment facilities have begun to replace metal parts with plastic because they are non-corrosive, energy efficient, light weight, less cost to install or repair, and have proven to be superior in wear and performance.